1. Field of the Invention
This invention relates to the field of computer display memories and, in particular, a display random access memory for use with a liquid crystal display (LCD) device.
2. Background Art
In a computer system, a display device is provided to view data entered by a user and to view the results of processing operations on that data. Often, a liquid crystal display (LCD) is utilized as the display device. The LCD has the advantage of being flat as compared to the substantial depth of a cathode ray tube (CRT) type display. Therefore, LCD's have particular application in situations where size is a factor, such as in portable or lap-top computers, hand-held televisions, watches, etc.
An LCD consists of an array of "cell" or "pixel" (picture element) locations. In a monochromatic display, each pixel location has two states, "on" or "off." In one state, the pixel is black, and in the other state the pixel is white. By selectively setting the state of certain pixels, the array of pixels, when viewed in combination, forms an image.
Each pixel or cell of the LCD consists of liquid crystal material disposed between two cell plates, a front plate and a rear plate. The liquid crystal material may be one of many well-known compositions, such as the ferro-electric liquid crystal known as DOBAMBC. The front plate (which is transparent) and the rear plate each support an electrode so that an electric field can be applied to the liquid crystal material in the cell. The liquid crystal material can be switched between two stable conditions by applying an appropriate electric field. Typically, a varying voltage is applied to each of the plates in order to switch the states of the liquid crystal material. Cancellation techniques are used to alleviate cross talk problems. By switching the states of the liquid crystal material, the cell can be made transparent (white) or opaque (black). Thus, the liquid crystal material can be used to generate a monochromatic display.
The voltage required to switch the state of a ferro-electric LCD cell is referred to as its "threshold" voltage V.sub.T. To switch the state of an LCD cell, a voltage greater than V.sub.T is applied to the cell. In other LCD systems, row access is accomplished via pulse width modulation (PWM) or by applying an analog voltage to charge a capacitor associated with the cell to an intermediate value.
The cells of an LCD are arranged in "rows" and "columns". In this specification and following claims, a row is a horizontal line containing a plurality of cells or pixels. Generally, a plurality of rows are defined from top to bottom on a display. A column is a vertical line containing a plurality of cells or pixels. A plurality of columns are defined from left to right on a display. Each row is assigned a row number, generally increasing from top to bottom, and each column is assigned a column number, increasing from left to right.
To access an individual cell or pixel, the row containing that cell is "strobed" with an "address" pulse. The column containing that cell is also strobed with a "data" pulse. The row address pulse and the column data pulse are individually less than the threshold voltage V.sub.T. However, the combination of the two pulses is greater than V.sub.T. In this manner, only a cell whose row number and column number is strobed is accessed, and all other cells in the row and column remain in their original state. Alternatively, the rows can be strobed with a data pulse and the columns with an address pulse.
An LCD is known as a "long persistence" display. That is, the display will retain a display image for a long period of time before a refresh cycle is required. By contrast, cathode ray tube (CRT) type displays must be refreshed frequently, or the display image will disappear.
A disadvantage of LCD devices is their relatively slow switching time when a pixel location location is changed from one state to another state. The slow switching time can be distracting to a user. Also, the cumulative effect of slow switching time for each cell location reduces the bandwidth of the display and the ability to display rapidly changing images. This can limit the effectiveness of an LCD in animation applications or other dynamic display contexts.
There have been prior art attempts to provide solutions to these problems by limiting the number of cell locations that are written or updated during a refresh cycle. Harada, et al, U.S. Pat. No. 4,693,563, is directed to a liquid crystal display (LCD) device and a driving method in which only the changing part of the display is refreshed. This is accomplished by refreshing those bands of rows that contain changed elements. A disadvantage of the device of Harada is that unnecessary updates are still required because portions of the bands of rows that contain the changed data are updated even though they do not require updating. In addition, Harada does not disclose a method of updating changes in non-consecutive single rows or columns of an LCD device. This limits the bandwidth of the device of Harada. Additionally, Harada does not disclose a method for identifying changed portions of a display.
Ayliffe, U.S. Pat. No. 4,703,305, describes a drive method for an LCD device that allows display data to be updated a whole row at a time or by a row segment. The device of Ayliffe is directed to the problem that can arise when typing data that is to appear on an LCD, namely the lag time in the data appearing on the display and the unwanted screen flicker that can accompany screen update for this new data. Ayliffe describes a method for defining the columns to the left of a character entry as "unselected columns." Only one column of cells is strobed for update or refresh and this column is sequentially advanced as characters are entered. This update scheme is effective if the key entry is less than 6.25 characters per second. If the entry rate exceeds this level, two characters are updated at one time resulting in a "buffering" effect to the user. Ayliffe does not suggest any method for refreshing non-consecutive columns. In addition, the system of Ayliffe requires a longer duration strobe pulse (at a lower voltage) for segment update than for whole row update. This further reduces the dynamic bandwidth of the display.
Crossland et al. U.S. Pat. No. 4,655,550 is directed to a ferro-electric LCD that allows data to be written to only those cells in each row whose state is to be changed while the remaining cells in the row are refreshed to maintain their existing state. In the embodiment described in Crossland, the existing state of the display is refreshed row by row while new data is written only to every N.sup.th row. N is an arbitrarily chosen number and rows to be updated in Crossland are not selected on the basis of whether or not the information they contain is being changed.
Starkey et al. U.S. Pat. No. 4,775,859 describes a drive method for long persistence cathodochromic CRT displays. "Skip words" associated with rows of video data stored in video RAM are used to skip rows that contain no data and for interleaving. In Starkey, lines are skipped because they contain no display data at all or because periodic updating (every second or third row for interleaving) is being implemented. Starkey does not disclose a system for identifying changed locations only for display update.
A number of other prior art patents describe systems for providing improved display output. For example, Ott, U.S. Pat. No. 4,286,320 describes a computing system with a RAM memory sub-system featuring a RAM controller with an automatically incrementing address counter. This auto-incrementing feature eliminates the need for the CPU to generate addresses for reading or writing successive blocks of data, thereby freeing up CPU processing time. Oguchi, U.S. Pat. No. 4,356,482 describes a method of refreshing video data stored in dynamic RAM when (because a magnified image is being displayed) only a portion of the video data is read for display and then rewritten to memory. During a first time period, the portion of the stored video data that represents the magnified images is read, displayed and rewritten to memory. During a subsequent second time period, the remaining video data is refreshed.
Two patents of Dixon, et al. namely, U.S. Pat. Nos. 4,489,378 and 4,490,782 describe memory caching systems in which the data stored in the cache includes data that has already been requested by the CPU as well as additional data that is likely to be requested in the future. Various algorithms are described for determining the makeup of this additional data based on the data actually requested.
Rajaram, U.S. Pat. No. 4,511,965, describes a method for prioritizing CPU and CRT controller access to VRAM. CPU access to VRAM is limited to periods between CRT controller access. Since the CRT controller is given priority access to VRAM, scanning of an image onto the CRT display is not interrupted by CPU access to VRAM. The quality of the image display on the CRT is thereby improved.
Roberts, U.S. Pat. No. 4,796,203, is directed to an interface that allows updated display information to be written simultaneously to a monitor and to a refresh memory. Dawson et al. U.S. Pat. No. 4,884,220, describes an apparatus for directionally scanning a rectangular matrix memory containing image display information to produce perspective and rotated views of the image stored in the matrix.
Sfarti et al. U.S. Pat. No. 4,912,658, describes a graphics controller that stores words from an X by Y array bit map in multiple, identical X by Y array memory chips. (If the bit map consists of 1K by 1K array of 16-bit words, 16 separate one megabit 1K by 1K memory chips are used). Each memory chip in this case stores one bit of each 16-bit word of the bit map in the physical location in the memory chip that corresponds to the logical location of the 16-bit word in the bit map. No translation of the address in the bit map to the physical memory address is required and individual bits can be modified without modifying the entire 16-bit word.
None of the prior art provides a solution to updating individual display elements in non-consecutive rows or columns of a long persistence display or describes systems for increasing the bandwidth of LCD type devices.